being maintained at that temperature by the pressure of steam communicating with the valve, might probably account for the expansive curve showing a greater pressure than the ordinary rule gave.

Mr. E. A. COWPER stated, that in an indicator diagram taken from an engine with no steam-jacket, the expansion curve was a little above the ordinary rule, at the end of the stroke. This, he considered, indicated that the latent and sensible heat of steam together amounted to a greater quantity in high-pressure steam, than in low-pressure steam.

Steam, or gases, in expanding, and so giving out power, lost heat. Part of the sensible heat became latent in the production of power, and this heat could only be recovered, by expending the power already produced, in again condensing the steam back to its original bulk, when the latent heat again became sensible. As, however, the general object was to produce power, this plan could not, of course, be entertained. Therefore, the heat which was sensible, and became latent by the expansion in producing power, was, in fact, the necessary theoretical expenditure of heat required to produce power; and no engine could be made to work by simply supplying the loss of heat by radiation.

He must contend, however, that the sensible heat that was left in the steam, or air, after it had been expanded, could be stored up in plates of metal, or wire gauze, to a considerable extent, and that these plates, so heated, might be used to heat fresh steam, or air, that was about to be used for obtaining power.

Mr. D. K. CLARK remarked, that in his experiments on locomotives, he had found that with cylinders in the smoke-box, the curve of expansion was very little above that given by Marriotte's law; but that when the cylinders were exposed, it differed materially. He thought that in the diagram alluded to by Mr. Pole, the steam-jacket had influenced the form of the

curve.

Mr. SIEMENS agreed with Mr. Hawksley's proposition, that an engine of his proportions could not give out any power. The valve between the two cylinders in Ericsson's engine was as necessary as it was between the boiler and cylinder of an ordinary steam-engine, and could not be replaced by the regenerator,

which had a very different office to fulfil.

Ericsson's engine

might indeed be compared to a steam-engine, in which the boiler was represented by the air-chamber between the two cylinders, and the feed-pump by Ericsson's pumping cylinder. Ericsson had the disadvantage of sacrificing two-thirds of his power to move the pump, but had the advantage of expending no latent heat to form his elastic medium. But it must be borne in mind, that Ericsson had to add to his air a larger amount of sensible heat, of which only a small proportion was really expended in expansion, and the remainder would go to waste, unless it was recovered by the regenerator. Nevertheless, the drawbacks to Ericsson's engine, on account of the great resistance of the pump, the small working pressure, the insufficiency of heating surface, and the working of a piston in a heated cylinder, were so great, that he thought no beneficial results could be expected from it.

Mr. JAMES STIRLING, through the SECRETARY, after alluding to the specification of his brother's, the Rev. Dr. Stirling's, original patent for an air-engine, in 1816, quoted by Mr. Leslie, said that the engine then described was nearly similar to that mentioned in the Paper read to the Institution in June 1845; the only difference was that it had but one air-vessel, with a piston working in the open end of it, and the interior airtight vessel, or plunger, was worked by a rod, passing through a stuffing-box, in the centre of the piston. In small engines, where the surfaces of the passages between the air-vessel and the plunger bear a large proportion to the volume of air to be acted upon, these surfaces themselves form a considerable portion of the regenerating process; but the specification described this passage as "partially filled with studs, or with wires wound round the plunger, for the purpose of heating and cooling the air more completely, and with less waste;" and a passage filled with wire gauze was actually tried in one of Dr. Stirling's earliest experimental engines.

He constructed an engine of this description, in 1818, for pumping the water from a quarry in Ayrshire, which work it performed very well, until, from the carelessness of the engineman, the bottom of the air-vessel was overheated, and being

Vide "Minutes of Proceedings," Inst. C.E., 1845, vol. iv., p. 348.

made of boiler plate, of a flat conical form, it was crushed down by the pressure of the heated air, and rendered useless. This engine did not work to the power expected, and it was feared, at the time, that the air-vessels, for engines of large power, would require to be of enormous size. The undertaking was, therefore, for the moment abandoned.

In 1824, it occurred to him, that the difficulties attending the large working parts might be got over, and the dimensions of the engine greatly reduced, by working with air of a higher density, produced by mechanical compression. This involved the necessity of a closed cylinder, or double-acting engine, with two air-vessels, in order that there might be no means of escape for the compressed air, except by the piston and plunger rods. Having made some experiments with a working model constructed on this principle, these improvements were patented in the beginning of 1827, conjointly by Dr. Stirling and himself. In 1828, an engine on this principle, with a cylinder 26 inches in diameter and 3 feet stroke and about 20 horse-power, was constructed at Messrs. C. Girdwood & Co.'s, Glasgow. It was then found, that the small part of the heat which the 'Regenerator' failed to extract from the air, in its passage to the cold end of the air-vessels, accumulated to such an extent, that the requisite difference of temperature, between the two ends of the airvessels, could not be kept up, and that the power consequently fell off considerably. This engine was again abandoned; and it was only after overcoming this last difficulty, by the refrigerating apparatus, described in his Paper in 1845, and after testing its efficacy for several months, on a small engine, with a cylinder 3 inches in diameter, and 12 inches stroke, which worked to two-horse power, that the present patent was taken out. A full account of the engines constructed under this patent, was given in the Paper before mentioned. From some difficulties experienced in the management of the furnaces, and in heating the air-vessels equally, the use of the engine there described, was discontinued about a year after the date of the Paper, after it had worked efficiently for more than three years. Still the subject was not abandoned; and he hoped, ere long, to bring it again under the notice of the Institution, with this, he might almost say, its only imperfection, entirely removed.

May 24, 1853.

JAMES MEADOWS RENDEL, President,

in the Chair.

The following candidates were balloted for, and duly elected : -William Richard Le Fanu, John Curphey Forsyth, and George Robert Stephenson, as Members.

No. 898.-" A Description of the Newark Dyke Bridge, on the Great Northern Railway." By JOSEPH CUBITT, M. Inst. C.E.

THIS bridge carries the Great Northern Railway across the Newark Dyke, a navigable branch of the Trent, at Newark.

The lines of the railway, and of the navigation, intersect each other at so acute an angle, that although the clear space between the faces of the abutments of the bridge is only 97 feet 6 inches, measured at right angles with the latter, yet the actual clear span of the bridge is 240 feet 6 inches.

There is nothing novel, or peculiar about the abutments. They consist of ordinary brickwork and masonry, and were founded on a bed of strong gravel, by means of coffer-dams.

The railway is carried across the space between the abutments by two separate bridges-one for each line of way—each bridge consisting of two trussed girders, constructed on Warren's principle, as developed by Mr. C. H. Wild, who first brought the principle under the Author's notice, in proportions and details nearly identical, in most particulars, with those ultimately adopted and carried into execution.

Each girder consists of a top tube, or strut of cast-iron, and a bottom tie of wrought-iron links, connected together by alternate diagonal struts and ties of cast and wrought iron respectively, dividing the whole length into a series of equilateral triangles, of 18 feet 6 inches length of side.

These girders rest on the apices of cast-iron A frames, placed on the masonry of the abutments. Each pair is connected by a [1852-53.]

2 R

horizontal bracing at the top and the bottom, leaving a clear width of 13 feet for the passage of the trains.

The top tube increases in diameter from 1 foot 1 inch at the ends to 1 foot 6 inches at the centre; the thickness of the metal also increases from 1 inch at the ends to 23 inches at the centre. Its total length is 259 feet. It is composed of twenty-nine separate pieces, each piece being accurately turned and fitted at its extremities, so as to insure the exact contact of two metal surfaces of the same area as the cross section of the tube. The pieces are connected together by eight bolts and nuts, passed through small projecting snugs.

Those lengths, or portions of the tube, to which the diagonal struts and ties are connected, are accurately bored, to receive the joint pins, at right angles to the axis of the tube, and are finished with accurately-faced fillets at the ends of the transverse holes.

The lower tie consists of wrought-iron links 18 feet 6 inches in length from centre to centre of the holes, each link being rolled in one piece without any welding. They are of the uniform width of 9 inches, but vary in number and thickness, according to the strains to which each length, or portion of the tie, is subject. Thus

The 1st, or end lengths near the abutments consist of 4 links

Inches. 9 x 1

The ends of the links are swelled laterally to the width of 16 inches, for the reception of the joint pins; the swelling is diminished very gradually down to the uniform parallel width of 9 inches. The diameter of the holes, for the joint pins, is 5 inches; the semi-circumference is about equal to the width of the links, and the thickness of the iron on each side of the hole is 5 inches.

The diagonal links are of precisely the same form and dimensions as those of the horizontal tie; and are, in like manner, adapted to the strains to which they are subject, by varying their number and thickness.